1. Field of the Invention
This invention relates to printing and more specifically to electrophotographic printing.
2. Description of Prior Art
Line printing can be divided into two areas; impact and non-impact printing. Impact printing relates to a device that has one or more character elements, a ribbon or other inking device, a paper transport and some capability of impacting the desired character elements on the paper transferring the physical image of the character element through the ribbon leaving an ink impression of the character element on the paper. Non-impact printing relates to devices that use other than mechanical means to transfer images onto paper. The area of non-impact printing includes direct electrostatic, magnetic, ink jet and electrophotographic printing. An example of direct electrostatic printing is contained in U.S. patent application Ser. No. 267,225, filed May 26, 1981, now abandoned. The electrostatic printer uses a dielectric drum which is selectively charged at certain pixel sites and then exposed to a toner with charger toner particles that are attracted to the charged pixel sites. The drum with charged toner then contacts paper in such a manner that the toner is transferred to the paper leaving a clean drum. Magnetic printing is similiar, except the drum contains magnetized pixel sites and the toner contains magnetic toner particles. An example of a magnetic printer is the 3M Magnastylus. Ink jet printing involves the production of a stream of ink directed at the paper. The stream is controlled by electrostatic acceleration and deflection of the ink particles to form characters on paper. The last non-impact printing area is electrophotographic printing. Electrophotographic printing is a xerographic process using light and a photoreceptor drum and toner to transfer characters to paper. The electrophotographic printers include a light source that is modulated to selectively project light upon a photoreceptor drum made of a photosensitive dielectric that has been electrically charged. Areas of the drum where light is projected become electrically neutral so that the drum then becomes selectively charged. The drum passes through a toner station where charged toner particles come into contact with the drum. The toner particles adhere to the drum at the charged areas and do not adhere to the uncharged areas. The drum then comes is contact with paper which is also charged. The drum then transfers the toner particles to the charged paper, reproducing the image to be printed on the paper. The toner is then fused to permanently adhere the toner particles to the paper.
Electrophotographic printing can also be divided into several areas including laser printing, liquid crystal display (LCD) printing, visible light emitting diodes (VLED) printing and printing using a cathode ray tube. Laser printers include solid state lasers and gas lasers. Solid state laser printers control the laser light by using either polygomirrors, acoustooptic scanners or a diffraction grating. Gas laser printers control the laser light by using acoustiooptic deflectors or rotating polygon mirrors. Manufacturers of polygon mirror laser printers include Cannon, Xerox and IBM. LCD printers are of two types: transmissive and reflective. The transmissive type controls the light from a separate light source which passes through the LCD device onto the photoreceptor drum. The reflective type is similar except light is reflected by the LCD device. LCD printers have been the subject of many papers such as "A New Electrical to Optical Image Converter--The CCD Liquid Crystal Light Value" by P. O. Braatz, et al, published in the 1980 Biennial Display Research Conference Proceedings, "A Fast Silicon Photoconductor--Based Liquid Crystal Light Value" by P. O. Braatz et al, published in the 1979 IEDM Proceedings and "A Facsimile Printer Utilizing an Array of Liquid Crystal Cells" by Juri Tults published in the Proceedings of SID Vol. 1214 Fourth Quarter 1971. One manufacturer of LCD printers is Static Systems Corporation. VLED printers include the fixed linear array type which project light through a line of VLED dots onto a rotating photoreceptor drum and the scanning type which further include scanning a limited aray by a raster motion. Printers using cathode ray tubes (CRT) include projection CRT's using self-focusing lenses and fiber optic CRT's. Manufacturers of CRT printers include Wang and Konishiroku.
An additional type of electrophotographic printer uses a deformable mirror display (DMD) projection system and is the subject of the present application.
A form of the deformable mirror display is discussed in a paper entitled, "A Membrane Page Composer" by L. S. Cosentino and W. C. Stewart, published in the RCA Review, Vol. 34, p. 45, March 1973. Another paper that discusses deformable mirrors is titled, "The Membrane Light Modulator and its Application in Optical Computers" by Kendall Preston, published in the OPTICA ACTA, Vol. 16, No. 5, pages 579-585, 1969. A form of the DMD is described in detail in U.S. patent application, Ser. No. 183,595. The object of the DMD projection printer is to provide at a moderate cost a printer that can print at the rate of 10 to 40 pages per minute on most plain paper with a resolution of 300 pixels per inch vertically and horizontally.